Inverters are used in photovoltaic systems for converting a direct current generated by a photovoltaic generator into alternating current, which can be fed, single-phase or polyphase, into a public or private power supply grid. In the context of the application, a photovoltaic generator, referred to below as a PV generator for short, should be understood to mean any arrangement comprising photovoltaic modules (PV modules), in particular, an arrangement in which a plurality of PV modules are connected in series to form a so-called string.
Inverters or more generally voltage converters of a PV system generally comprise a tracking device for the working point of the PV generator connected to them, the so-called MPP (Maximum Power Point) tracker. By means of the MPP tracker, the PV generator is operated at a working point at which the maximum electrical power is output. This optimum working point, also referred to as MPP working point, is dependent on the respective PV generator and its operating parameters, for example the intensity of the solar radiation and the temperature of the PV generator. Furthermore, the working point shifts towards lower voltages over the course of the life of PV generators as a result of their aging response (degradation).
A summary of various MPP tracking methods can be found in the article “Comparison of the performance of maximum power point schemes applied to single-stage grid-connected photovoltaic systems”, S. Jain and V. Agarwal, IET Electr, Power Appl., 2007. In the case of a frequently used tracking method, the voltage at the input of the voltage converter, which is also referred to as the intermediate circuit voltage in particular in the case of single-stage inverters, i.e. inverters which only have a converter stage in the form of an inverter bridge, is increased or decreased by increments, wherein, with each increment, a change in the power generated by the PV generator and converted by the inverter which is associated with this increment is determined. As long as the power increases, the change in voltage generally goes in the direction of the optimum working point. Correspondingly, for a subsequent variation increment, the direction of the change in voltage (increase/reduction) can be maintained. If, on the other hand, the power is reduced in the event of a variation in the intermediate circuit voltage, the direction of the subsequent increment can be reversed. In this way, the intermediate circuit voltage follows or fluctuates around the voltage at the optimum working point.
A modification to such a tracking method is known from the document U.S. 2005/0110454 A1. In this case, it is not the power that is generated by the PV generator that is considered, but the power which is output by the inverter of the PV system. The document DE 19961 705 A1 describes an inverter, in which the working point of the PV generator is set both in respect of maximization of the output power of the PV generator and in respect of the level of the output voltage of the inverter.
In the case of single-stage inverters, a change in the working voltage of the PV generator is performed by a variation in the voltage conversion ratio between the input-side intermediate circuit voltage and the output-side mains voltage of the inverter bridge. Such a variation in the voltage transformation ratio can be performed, for example, by different switching parameters of switches in the inverter bridge (for example switching duration, duty factor, phase angle of the switching operation in relation to the phase angle of the AC voltage). The variation range of the voltage transformation ratio is in this case relatively restricted.
In particular, a lower voltage limit is provided for the intermediate circuit voltage of an inverter or voltage converter which needs to be exceeded in order to establish a power flow from the PV generator into the energy supply grid.
It is known from the prior art, for example, from the document EP 2 107672 10 A1, to determine the minimum required intermediate circuit voltage of the inverter and therefore the lower input voltage limit of a single-stage inverter computationally on the basis of a measured voltage of the energy supply grid and the known maximum voltage transformation ratio. In practice, known MPP tracking methods do not, however, use the working voltage range up to this computationally determined lower input voltage limit. In the document EP 2 107 672 A1 mentioned above, for example, a control reserve is described which is added to the computationally determined lower input voltage limit. The reason for this is that, in the vicinity of this limit, for example, owing to the voltage regulation mechanisms of the input voltage, there is the risk of the minimum required intermediate circuit voltage temporarily being undershot, which would result in distortions in the signal waveform of the fed-in alternating current. Therefore, the minimum required input voltage has a reserve applied to it in order to safely avoid such distortions.